This invention generally relates to fluorescent lamps and, more particularly, to an improved fluorescent lamp of warm-white color which has both good efficacy and good color rendering properties.
Fluorescent lamps are fabricated to provide various color temperatures of emission which generally correspond to the color temperature of a complete or full radiator maintained at the specified temperature. A warm-white color is specified as being about 3000.degree. K. and cool-white color is specified as being about 4100.degree. K. For illumination in the home, the public seems to prefer the warm color, possibly due to long-time familiarity with the incandescent lamp which has a warm color.
A standard warm-white fluorescent lamp designed to operate with a color temperature of about 3,000.degree. K. is normally fabricated with a halophosphate phosphor, such as generally described in U.S. Pat. No. 2,488,733, dated Nov. 22, 1949, to McKeag et al. More specifically, such a phosphor is normally apatite-structured calcium fluoro-chlorophosphate, activated by antimony plus manganese, preferably with a small addition of cadmium. Such lamps have reasonably good lumen output, a representative output for a 40WT12 lamp being 3100 lumens, but the color rendering properties of such lamps are relatively poor. Fluorescent lamps having a warm-white color and relatively good color rendering properties are available on the market, and these lamps incorporate a phosphor comprising about 90% by weight strontium magnesium phosphate activated by tin, 5% by weight manganese-activated zinc silicate, and 5% by weight of a blue halophosphate, which is calcium fluoroapatite activated by antimony. The lumen output of these lamps is relatively poor, however, and a considerable sacrifice in lumens is made in order to obtain the good color rendering properites.
The internationally accepted procedure for standardizing and measuring the color rendering properties of light sources is set forth in the publication of The International Commission on Illumination, identified as publication C.I.E. No. 13 (E-1.3.2) 1965. More recently, a color-preference index has been proposed for rating the performance of light sources in accordance with what the normal observer considers to be the preferred coloration for familiar objects. This color preference index (CPI) is summarized in the Journal of the Illuminating Engineering Society, pages 48-52 (October 1974) article entitled "A Validation of the Color-Preference Index" by W. A. Thornton.
In U.S. Pat. No. 4,079,287, dated Mar. 14, 1978 to Soules et al is disclosed a fluorescent lamp which utilizes a mixture of strontium chloro-fluoroapatite activated by antimony and manganese and yttrium oxide activated by tervalent europium, in order to obtain an improved efficacy as compared to a so-called deluxe-type lamp, while still providing a good color rendering index. The lumen figures which are reported, however, are inferior to the standard fluorescent lamps which use halophosphate phosphor and which have a lower color rendering index.
In U.S. Pat. No. 4,075,532, dated Feb. 21, 1978 to Piper et al. is disclosed a fluorescent lamp having a cool-white color which utilizes a mixture of calcium fluoroapatite activated by antimony and manganese, which is a broad-band emitting phosphor having an emission peaked in the yellow region of the visible spectrum, together with strontium chloroapatite activated by divalent europium, which is a narrow-band emitter peaked in the blue-violet region of the visible spectrum. The phosphors are mixed in predetermined proportions in order to provide a cool-white emission which has improved luminosity over the standard halophosphate phosphor, but with a somewhat decreased color rendering index.
In U.S. Pat. No. 3,602,758, dated Aug. 31, 1971 to Thornton et al is disclosed a fluorescent lamp which utilizes a double layer of phosphor, with the more expensive phosphor positioned nearest to the discharge to cause it to "work harder" so that less of this more expensive phosphor will be utilized. More specifically, the phosphor which is furthest from the discharge is a mixture of manganese activated zinc silicate, strontium blue halophosphate and strontium magnesium phosphate activated by tin, with a relatively expensive yttrium oxide activated by tervalent europium coated thereover.